1. Field of the Invention
The present invention relates to phorbol derivatives having antiviral activity, more particularly to an antiviral agent effective against virus such as human immunodeficiency virus (HIV) containing a phorbol derivative as an active ingredient, and to novel phorbol derivatives.
2. Description of the Related Art
Since the discovery of human immunodeficiency virus, the causative agent of the acquired immunodeficiency syndrome (AIDS), significant progress has been made towards the development of effective anti-HIV drugs and recently considerable progress has been made in this field. In the research and development of AIDS therapeutics, research for natural substances having anti-HIV activity has been made besides research and development of novel chemical drugs. For example, a variety of compounds having various chemical structures derived from plants have been reported to be effective in inhibiting the replication of HIV-1 or its essential enzymes (cf. for example, Che, 1991; Schinazi, 1992; Nasr, Cradock & Johnson, 1992; El-Mekkawy et al., 1995; El-Mekkawy, Meselhy, Kusumoto, Kadota, Hattori, Namba, 1998; Ng, Huang, Fong & Yeung, 1997; Kusumoto & Hattori, 1999).
Plant-derived bioactive substances are relatively easily available from plants. There are many plants which are used as materials for Japanese-Chinese medicines or folk medicines over the world so that many pieces of information on bioactivity have accumulated and the plants are greatly expected to be also potent anti-HIV drugs.
However, none of the plant-derived compounds that have anti-HIV effects is sufficient in its activity. Since some of the above-mentioned plant-derived substances show deleterious side effects such as cytotoxicity or carcinogenicity, it is very difficult to select most suitable antiviral agents such as, for example, anti-HIV drugs, taking into consideration both useful bioactivity and harmful bioactivity. Therefore, it has been keenly desired to find plant-derived bioactive substances having high antiviral effects (for example, anti-HIV effects) and having less deleterious side effects and develop effective antiviral agents, for example anti-HIV agents, based thereon.